Python __sizeof__() Method

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Syntax

object.__sizeof__(self, other)

The Python __sizeof__() method returns the size of the object in bytes. The sys.getsizeof() method internally call’s __sizeof__() and adds some additional byte overhead, e.g., for garbage collection.

Basic Example

The following code snippet creates a list object x with three integers and measures its size in bytes (104) using the x.__sizeof__() method call.

It then measures the size with overhead using sys.getsizeof(x).

>>> import sys
>>> x = [1, 2, 3]
>>> x.__sizeof__()
104
>>> sys.getsizeof(x)
120

Let’s make the list much larger to measure if it has any impact on the size in bytes.

>>> import sys
>>> x = list(range(10000))
>>> x.__sizeof__()
80040
>>> sys.getsizeof(x)
80056

This list is built using the range() function. It has 10000 integers, each needing 8 bytes with a constant overhead of 40 bytes for the list structure, the x.__sizeof__() method call results in 10000 * 8 + 40 = 80040 bytes.

You can see that in both cases, the sys.getsizeof(x) has 16 more bytes than x.__sizeof__() which seems to be the overhead on my particular machine for this particular example.

Overriding __sizeof__()

You can override the __sizeof__() method for your own custom data type by defining the __sizeof__() method with one argument self (that is passed automatically by Python).

import sys

class Data:
    def __sizeof__(self):
        return 42
    
    
x = Data()
print(x.__sizeof__())

The __sizeof__() method returns the integer 42 that was returned by us—not the number of bytes.

Let’s call the sys.getsizeof() method to see if the difference is 16 bytes for the GC overhead!

import sys

class Data:
    def __sizeof__(self):
        return 42
    
    
x = Data()

print(x.__sizeof__())
# 42

print(sys.getsizeof(x))
# 58

References:

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